Solid-liquid continuous countercurrent purifier method and apparatus

ABSTRACT

At least one component of a multicomponent molten mixture is separated from the mixture and purified by introducing the mixture into the recovery section of an array of apparatus comprising a recovery section, a refining section and a purifying section. The recovery and purifying sections each have helical scraper-conveyors to move crystals through them, increasing cross sectional areas in the direction of crystal progression and downward temperature gradients in the direction of liquid and reflux movement which is countercurrent to the crystal progression. The feed mixture is introduced to the recovery section near its junction with the refining section and flows in the countercurrent or reflux direction. Crystallization occurs at a certain temperature encountered in the recovery section and the crystals grow and are purified as they are moved by the scraperconveyors through the recovery and refining sections and into the purifying section and purify by gravitational movement through the countercurrent molten reflux. At the end of the progression through the purifier, the crystals are remelted and withdrawn as substantially pure preselected component.

United States Patent Brodie Feb. 29, 1972 SOLID-LIQUID CONTINUOUSCOUNTERCURRENT PURIFIER Primary Examiner-Norman Yudkofi' AssistantExaminer-S. Silverberg HO N APPARATUS figtogney-Paul A. Rose, Louis C.Smith, Jr. and Maurice W.

a [72] Inventor: John Alfred Brodie, New South Wales,

Australia [57] ABSTRACT Assigneei Union Cirbide Australia Limited Atleast one component of a multicomponent molten mixture [22] Filed: Sept.17, 1969 is separated from the mixture and purified by introducing themixture into the recovery section of an array of apparatus 1 PP N05358,596 comprising a recovery section, a refining section and apurifying section. The recovery and purifying sections each have [30]Foreign Applicafion Priority Dam helical scraper-conveyors to movecrystals through them, in-

v creasing cross sectional areas in the direction of crystal Sept. 18,Australia progression and downward temperature gradients in thedirection of liquid and reflux movement which is countercur [52] US. Cl...23/273 F, 62/58 rem to the crystal progression. The f d mixture isintroduced [51] f" "Bold 9/04 to the recovery section near its junctionwith the refining sec- [58] Field Of Search "62/58; 23/273 F, 273 tionand flows in the countercun-em or reflux direction Crystallizationoccurs at a certain temperature encountered in [56] References cued therecovery section and the crystals grow and are purified as UNITED STATESPATENTS they are moved by the scraper-conveyors through the recovery andrefining sections and into the purifying section 2,617,273 I 2 Findlay Fand purify by gravitational movement through the countercur- 2,617,274 11952 Schmidt F rent molten reflux. At the end of the progression throughthe 2,679,539 5/ 1954 McKay F purifier, the crystals are remelted andwithdrawn as substan- Gunness F tially pure preselected c mponenL3,375,082 3/1968 Graf ..62/58 12 Claims, 6 Drawing Figures 76 v I -i 15i l I 1 I I J PAIENTEDFEBZQ I972 3,645,699

SHEET 1 [IF 5 INVENTOR JOHN A. BROD/E W I I 4 ATTORNE PATENTEUFEB 29I972 SHEET 2 BF 5 INVENTQR JOHN A. BROD/E 7 [4V ATTORNE% J BE-PAIENTEDFEB29 m2 3, 645.699

SHEET 5 [1F 5 I F/G.5. F0

INVENTOR JOHN A. BROD/E SOLID-LIQUID CONTINUOUS COUNTERCURRENT PURIFIERMETHOD AND APPARATUS This invention relates to a solid-liquid continuouscountercurrent separation and purification method and apparatusemploying crystallization phenomena.

It is known in the materials separation arts that where close boilingpoints of materials in mixture make for great difficulty in separatingsuch materials by distillation techniques, provided the melting pointsof the materials in mixture are sufficiently spaced to render itfeasible, resort to crystallization procedures may permit separation ofa solid phase from a liquid phase with greater facility than and oftenwith great advantage over distillation techniques.

An elaborate technology has evolved utilizing combinations ofcrystallizers and purifiers, and with the optional addition of solventmaterials, to achieve separations with crystallizing procedures. Resorthas been made to sieves or screens, to porous pistons, to pulsedoperation and to a variety of other devices for moving crystals andliquids. While significant advances have been made in such technology,nov completely satisfactory crystallization process has been madeavailable to industry. It is known for instance to those skilled in theart that the achievement of a 99 percent pure desired product is oftenrelatively simple compared with the critical operation and effortrequired to achieve 99.99 percent purity in commercial operation witheconomic throughput. Indeed, there are a number of reports in therelevant literature indicating that scale-up from bench size to pilotplant installations is far from easy, and thus far some of the problemshave proved insuperable.

With this then being the state of art, the present invention was made toprovide a process and means to achieve separations of components frommulticomponent mixtures through crystallization procedures.

It is an advantage of the present invention that by itcommercialoperation to give purities in excess of 99.99 percent can beachieved.

It is a further advantage of the present invention that it is operatedwith high thermodynamic efficiency, notwithstanding the fact that theequipment is of comparatively simple construction. Said high purity andhigh efficiency are believed to arise from the novel features of theprocess reflected in the equipment design, which depart significantlyfrom those taught by the prior art.

It is a still further advantage of the invention that the design andoperating parameters are so related that they may be readily adapted tocomputerized control if desired, though it must be recognized thatconsiderable change in certain design parameters of equipment will benecessary if there is a radical change in feed stocks or products. Wherefeed stocks suffer from minor fluctuations, operating parameters can beadjusted to achieve equilibrium operation; larger fluctuations may bedealt with by minor changes, such as alteration of point of feed stockentry, or the incorporation of comparatively simple surge absorbingzones. Gross changes will call for design variation, which however neednot involve major rebuilding, as will become apparent hereafter in thediscussion of forms of equipment that can be employed in practicing theinvention.

Another advantage of the present invention lies in the fact that itallows a complete separation of one of the components of the feed stock,within the limits set by an eutectics, in one totally sealed continuouspiece of equipment. This is of extreme importance where hazardous ortoxic substances are involved.

While the invention is primarily described as separating mixtures oforganic materials, it can be adapted to solutions wherein an equilibriumliquid phase is substituted by a saturated solution, 'which may includean aqueous solution of a substantially nonfusible inorganic salt.

In general according to the invention there is provided a process andapparatus for the separation in very pure form of at least one componentof a multicomponent mixture whereby the mixture is fed into asolid-liquid continuous countercurrent purifier, which includes of apurifying section, a refining section and a crystal-forming recoverysection, all in series. The mixture is fed into the recovery station ata point near the junction of the recovery section and the refiningsection. A continuous downward temperature gradient is provided from thejunction of the refining section with the purifying section to the endof the recovery section remote from the purifying section by continuousheat extraction throughout the length of said sections which are fittedwith cooling jacket or the like means. A liquid phase velocity at anypoint in a direction opposite to the direction of crystal movement ismaintained which velocity is greater than the back mixing velocity ofliquid at said point under the influence of agitation, crystaltransport, and convection instability. The crystal phase is maintainedin suspension in the liquid phase in each section in a stateintermediate between sedimentation and fiuidization by the control ofagitating means and liquid flow velocity. Where necessary to preventcrystal buildup on unscraped surfaces in the recovery and refiningsections, all such unscraped surfaces are provided with a sufficientsmall positive heat input. The purifying section is operated underessentially adiabatic conditions modified only by a small heat inputthroughout its length to keepthe temperature of its wall and agitatingmeans just above the crystallizing point of the liquid immediatelyadjacent thereto. Crystals are transferred slowly from the crystalforming recovery section through the refining section to the purifyingsection to maximize equilibrating crystal contact with thecountercurrent flow of liquid and to cause adequate growth and purity ofcrystals finally fed into the purifying section.

The arrangement of a purifying section a refining section and a crystalforming recovery section in series, with a feed point near the junctionof the refining section and the recovery section, a pure product outputat the end of the purifying section remote from said feed point and asecond product or mother liquor output at the end of the recoverysection remote from said feed point represents a development of thecenter-fed type crystallization apparatus. Its construction andoperation have distinct differences from known types end-fedcrystallization apparatus which generally feature a crystal formingsection and purifying section operating without the limitations andstructures of the process of the present invention. In particular it isto be noted that the endfed crystallization apparatus usually employflash chillers to cool the mix introduced to the purifying section, saidchillers having their coolant circulation so arranged that there existsan upward temperature gradient from the junction of the chiller with thepurifying section to the feed point remote from the purifying section;that is, the end-fed crystal forming section operates with alow-temperature feed to the refining or purifying section.

It is an important feature of the present invention that it operateswith a continuous downward temperature gradient (not necessarily linear)from the junction of the refining section with the purifying section tothe feed input point and continuing from the feed input point throughthe length of the recovery section to the liquid discharge point at theend of the recovery section most remote from the purifying section. Thusthe countercurrent purifier of this invention operates with a feed at atemperature practically identical to the temperature obtaining in theapparatus where it enters close to the junction of the refining andrecovery sections, which temperature is intermediate of that of the lowtemperature liquid discharge from the remote end of the recovery sectionand the maximum temperature in the refining section where reflux liquidenters from the purifying section. It is further to be understood thatthis downward temperature gradient through the length of the refiningsection, when associated with the slow crystal transport and maximizedequilibrating contact provided by the invention, means that crystalstransported through the refining section pass in succession through aseries of equilibrating stages characterized by increasing temperature:consequently the length of the refining and recovery sections and thetemperature difference between their remote ends will determine thenumber of equivalent plates" available in these sections for thepurification of a mixture in terms of a liquid-solid composition whosetemperature and composition can be represented on an associated phasediagram.

In the practice of the invention, the downward temperature gradient inthe direction of liquid flow within the sections of the apparatusrequires that the coolant circulation moves in a direction opposite tothat of the flow of the liquid phase of the mix with the consequentbenefit of enhanced thermodynamic efficiency. One or more coolantcircuits may be employed, according to required temperature and heatloads required. Separate control of the countercurrent coolant to therefining and recovery sections, for instance, makes it possible toattain preselected temperature gradients downwards from the end of therefining section contiguous to the purifying section and upwards fromthe remote or liquid discharge end of the recovery section so that thetwo gradients meet at the point of feed stock inlet to produce atemperature there equal to the crystallization point of the product ofinterest. In operation, the liquid phase flow is in a direction oppositeto that of crystal movement, and of a velocity that exceeds the backmixing velocity of the liquid under the influence of agitation, crystaltransport and convection instability. Known devices employ agitators orconveying means which operate at relatively high speeds. This causesturbulence and a back-mixing effect which tends to negate the progressin purification achieved in the crystal phase. Since the passage of thescraper-conveyor blades through the liquid crystal mixture inherentlycauses a certain amount of backmixing and, further, since there isalways a tendency for a liquid to adhere in layer formation to crystalssuspended in it, backmixing cannot be completely avoided. Again, in somecrystallization separation processes there is the possibility that afeed stock, denuded of one component by crystallization and perhaps alsoaffected by a temperature reduction, may have a higher density than theoriginal or initially introduced form or a partially denuded formthereof. Such more dense liquid may, in certain arrangements ofapparatus, tend to graviate undesireably into zones holding comparativepure product.

The selection of agitator and scrapenconveyor rotational speeds withattendant control of liquid phase flow velocities however, as providedby the present invention, minimizes these undesireable backmixingtendencies.

There is, however, one difficulty in insuring that the liquid phasevelocity shall always exceed a certain minimum figure. Thecrystal-liquid mixture must have crystal removed from it for the processto operate and the liquid phase is continually reduced by deposition ofcrystals therefrom under the influence oflower temperature.

The apparatus of the invention is therefore designed, fabricated andoperated to insure that the desired liquid phase velocity can bemaintained throughout the length of the refining section and therecovery section. This is achieved by providing refining and recoverysections each having decreasing cross sectional areas in the directionof fluid flow. Each such section may be made in tapered form or,alternatively of a series connected array of cylindrical elements eachhaving a cross-sectional area smaller than the preceding element alongthe fluid flow direction. The larger cross sectional areas in each ofsuch sections are associated with relatively higher temperaturesrelatively larger liquid-crystal mix volumes while the smaller crosssectional areas are associated with relatively lower temperatures, andrelatively smaller liquid-crystal mix volumes.

It is of importance in practicing the invention that equilibration mustbe maximized. Therefore, in addition to control of liquid back mixing,means for the control of the crystal phase is provided. lf the crystalphase sediments, there is inadequate exchange and equilibration with theliquid phase. If the crystal phase becomes fluidized, there will beexcessive back-mixing of crystal, and loss of through-put. It is knownthat there is a condition with solid-liquid systems which isintermediate between sedimentation and fluidization, where the bed ofsolid particles swells, but the solid particles are not able to movefreely through the expanded solid bed. It is therefore necessary tocontrol the agitating and scraper-conveyor means and the liquid flowvelocity in each of the recovery, refining and purifying sections 50that the crystal phase is held in suspension in the liquid phase in astate intermediate between sedimentation and fluidization. ln conformitywith this requirement, an agitator and/or scraper conveyor may in agiven case operate at the slow speed of half a revolution per minute,which contrasts strongly with agitator speeds described in some of theprior art.

One particular form of crystal agglomeration is encountered wherecrystal masses grow on unscraped surfaces, such as shaft bearings,sections of agitator shafts, the flat sides of scraper-conveyor bladesas these are cooled by thermal conduction to other sections of theapparatus at lower temperatures, or, in the case of scraper-conveyorblades, as these are cooled by mechanical contacts with the cooledsurfaces of vessel walls or layers of crystaline solids attachedthereto. All such accumulations hinder the proper operation of theequipment employed in the invention, and since it is important thatagitators and scraper-conveyors move slowly, it is unlikely shouldsuchcrystal masses disengage that they will be broken up in order toeffect equilibration.

A significant integer of the invention is therefore the requirement thatthe operation shall allow, where necessary to prevent such crystalbuildup, a small evenly distributed heat input to such unscrapedsurfaces. Electrical tracing with power led in through an agitator shaftis an effective means of achieving such heat input.

It is known to generate a reflux in a vertical purifying section byapplying heat to the discharge end of such purifying section, to causemelting withdrawing a proportion of melt and returning the balance ofsaid melt countercurrent to a descending bed of crystals as a reflux.Some end-fed crystallization columns of the prior art discharge suchreflux below the top of the purifying section by means of sieves orporous pistons. The present invention operates by returning reflux tothe refining section. Certain end-fed crystallization columns of theprior art employ a low temperature feed to the purifying section. Thepresent invention employs a high temperature feed to the purifyingsection. Certain end-fed crystallization columns of the prior artcompact the crystal bed in the purifying section with pistons, andothers apply pulsed agitation. The present invention maintains a crystalbed in the purifying section in a state intermediate betweensedimentation and fluidization, preferably allowing the crystal bed toform under gravity, and to reform after the small disturbance producedby a very slowly revolving agitating means and the countercurrent flowof liquid reflux. The present invention further pro vides that thepurifying section operates adiabatically, modified only by a small heatinput at least to keep the temperature of the wall and agitating meansabove the crystallizing point of adjacent liquid. This includescompensation for heat lost by conductivity along the purifying sectionwall and agitating means.

A gravity-operated purifying section performs best with a comparativelylarge crystal size. In general, large crystals are not grown by shockcooling or steep temperature/time gradients. For a given rate of crystaltransport, a temperature/time gradient can be transformed into atemperature/length gradient. In the practice of the present inventiontherefore, crystals which form in the recovery section pass only slowlythrough it and the refining section. This insures that the crystals aresubjected to a relatively small temperature/time gradient. Wherenecessary, the recovery and refining sections are designed to besufficiently long so that maximized equilibrating contact of crystal andliquid through these zones of relatively small temperature/timegradients is achieved. The consequent melting, recrystallization,remelting and further recrystallization steps then insure a feed ofadequate crystal purity and size passing into the purifying section fromthe refining section.

It is known in the crystallization separation art that impurities arecarried on crystal surfaces both by occlusion in crystal surface faultsand by migration into dendritic or the like features in crystal facia.It will be readily apparent to persons familiar with the art that thepresent invention as thus far described provides inherently for theremoval of such impurities with consequent enhancement of crystal puritythrough the optimum time shearing action obtaining between the liquidand crystal phases in counterflow relationship.

'The point of entry for the multicomponent feed stock is selected sothat its entry into the system effects minimum thermodynamicdisturbance. This involves a comparison of the composition-temperatureconditions obtaining within the continuous countercurrent purifier, andthe composition'temperature conditions in the feed stock when enteringthe purifier. Thermal stock'of the system by the entering feed stockwill thus be minimized.

The invention will now be described with greater particularity and withreference to the drawings wherein:

FIG. 1 is a partly sectionalized elevational view showing an apparatusarrangement which includes a recovery section, a refining section and apurifying section according to the invention.

FIG. 2 is a sectional view taken along the line 22 of FIG. 1, showingthe interiors of the purifying section and the refining section at itsjunction with the purifying section.

FIG. 3 shows an alternative partially sectionalized arrangement ofapparatus utilizing a multiplicity of cylindrical elements each toconstitute the recovery and refining sections according to theinvention.

FIG. 4 is a sectional view taken along the line 4-4 of FIG. 3 showingthe interiors of two elements of the refining section of the apparatusof FIG. 3.

' FIG. 5 is an elevational sectionalized view of an alternativearrangement of apparatus according to the invention involving acompletely vertical disposition of the recovery, refining and purifyingsections.

FIG. 6 is a schematic or block diagram illustrating a variation ofapparatus elements according to the invention.

One arrangement of the equipment of this invention is illustrated inFIGS. 1 and 2.

A refining section 1 and a recovery section 2 arranged end to end on onesubstantially horizontal axis are coupled to a vertical purifyingsection 3. Helical scraper-conveyors 4 and 5 are provided in therecovery section and refining section respectively, and slowly revolveto urge precipitated crystals towards the purifying section, whileproducing minimum back-mixing of liquid which moves in a directioncountercurrent to the crystal progression from the purifying section 3through the refining section 1, and, together with some feedstock fromits admission point, through the recovery section 2. Both the refiningsection 1 and the recovery section 2 are shown as uniformly taperedcylindrical vessels, but in altemative form they each may be built of asuccession of cylindrical vessels of decreasing diameter as outlined at6, 7, 8 and 9.

Cooling jackets l0 and 11 are provided for the refining section 1 andthe recovery section 2, the coolant entering at 12 into the jacket 11,thereafter passing into jacket via the bridging connection 13, andleaving jacket 10 at 14.

The helical scraping-conveyors are mounted on a shaft 17 supported bybearings and 16, and articulated at 31, and provision is made to supplya small heat input to the shaft 17, the scraper-conveyor blades 4 and 5and the spokes 18 to prevent crystal buildup on these unscrapedsurfaces.

The discontinuity in diameter which marks the junction 19 of therecovery section 2 and the refining section 1 is designed for maximumperformance to be adjacent to the feed inlet point 20. A liquid productis discharged at 21 at a relatively low temperature achieved in theoperation of the equipment. Although as illustrated in the drawings theapparatus is for a liquid feed stock, feed stocks which are themselvescrystalline can be processed according to the invention. For a liquidfeed as shown, the feed stock enters the apparatus at inlet point 20 onthe recovery section. Should a crystal feed be used, the inlet point 20would be located on the refining section I, again just adjacent therefining section/recovery section junction 19.

The purifying section 3 is contiguous to and connected with the largerend of the refining section 1. A weir 22 which may be provided withmeans to adjust its height relative to the shaft 17 serves to retain inthe refining section I a body of crystals which are'slowly transferredto the purifying section 3 as the scraper-conveyor 5, rotating clockwiseas shown in FIG. 2, raises the crystals to the edge of weir 22, overwhich they fall into the purifying section 3.

The purifying section is fitted with a slowly rotating stirrer 23mounted on a shaft 24 supported in bearings 26 and 27 carrying blades 25which are designed to prevent agglomeration of the crystal bed formingin the body of the purifying section 3 while minimizing turbulence andback-mixing. The insulated wall 28 incorporates heating means to providea small heat input just sufficient to keep the wall temperature at eachpoint above the melting point of the crystals accumulated adjacentthereto in effect compensating for the conduction of heat upward throughthe vessel wall toward the colder end 26 of the purifying section andoffsetting heat losses through the outer insulation.

Similarly through the shaft 24 there are incorporated heating means toprovide a small heat input to the shaft 24 and the blades 25 of thestirrer 23 for the same purpose.

One or more inspection ports 32 are an advantage in the purifyingsection construction.

Heating means 29 are provided at the base of the purifying section 3 toprovide the heat of fusion for the melting of the mass of crystals whichcontinuously reach the base of the purifying section. A portion of themolten material can be withdrawn through an outlet 30, the balance ofthe molten material being displaced upward through the purifying section3 by the descending mass of crystals of higher specific gravity. Theheat furnished by heating means 29 must be sufficient to melt thecrystals adjacent thereto at the bottom of the purifying section andthus permit liquid product withdrawn through outlet 30 but must not beso great as to cause increase in the relative velocities of thedescending crystals and/or the rising liquid reflux stream at any pointin the purifier section.

This arrangement of the equipment operates in the following manner.

Coolant at an appropriate temperature and in appropriate quantity iscaused to pass through the jacket system 12- l 1 13-10-14. Themulticomponent feedstock enters the recovery section 2 at the point 20,and under equilibrium conditions it is preferred that the feedstocktemperature be at the crystallizing point if liquid or for a crystalfeed at the melting point, which temperature in either case shouldclosely approximate the temperature of the solid-liquid mixture withinthe apparatus adjacent to the feed inlet point 20. This provision willprevent partial melting of the already-formed crystals and avoid shockcooling and excessive fine nucleation at the point of feedstock entry.

The feedstock will tend initially to pass toward the end 15 of therecovery section, and because of the temperature gradient will depositan increasing amount of solid crystal which will be conveyed toward therefining section, leaving a reduced amount of liquid to pass to thedischarge point 21.

There will be an optimal interrelation of the heat transfer, theliquid-solid ratio and the taper of the recovery section for eachmulticomponent system fed into such equipment.

The refining section 1 receives the crystal deposited in the recoverysection 2 by reason of the rotation of the scraperconveyor 4. Therefining section also receives a liquid component of high purityreturning from the purifying section 3 over the weir 22. The temperaturegradient through the refining section from 19 to 16 causes the crystalsbeing carried by the scraper-conveyor 5 to be subject to melting orpartial melting; on the other hand, the same temperature gradient causesthe liquid component returning from the purifying section 3 over theweir 22 into the refining section 1 to be subject to crystallization.

In the end result, the temperature gradient of the refining sectiontogether with the reflux into the refining section from the purifyingsection combine to produce a quality gradient in the refining section.

It is further to be noted that the arrangement described whereby thecoolant passes countercurrent to the liquid moving through the refiningand recovery sections permits high thermal efficiency, and in practiceis the opposite of most systems of the prior art.

The crystals lifted over the weir 22 by operation of theconveyor-scraper fall by gravity through the purifying section 3 and theblades 25 of the stirrer give a slowly, falling unagglomerated crystalbed through the depth of the purifying section, the level of crystal bedbeing maintained just below weir 22.

For liquid-solid systems in which the solid is of lower specific gravitythan the liquid, i.e., in which the solid floats in the liquid, it willbe sufficient to make a few minor modifications in adapting theapparatus just described. The purifying section will rise above therefining section, the weirs will control a bed of crystals floating onliquid.

As an alternative to the end-to-end arrangement of refining and recoverysections, a cascade arrangement is shown in FIGS. 3 and 4. In thisarrangement, the succession of three jacketed cylinders la, lb and 1c ofreducing diameters constitutes the refining section, and the successionof three jacketed cylinders 2a, 2b and also of reducing diametersconstitutes the recovery section. FIG. 4 represents a cross section inthe plane of the line 44 of FlG. 3 showing the connection between thecylinders lb and la. A scraper-conveyor 43 lifts crystals from thecylinder lb over an adjustable weir 45, whence they fall downward intothe cylinder la, to be conveyed by the scraper-conveyor 44 toward thepurifying section 47. An inspection port 46 is provided.

The feedstock 48 inlet is placed in proximity to the junction of therefining section 1c and the recovery section 2a. The liquid dischargepoint 49 is provided at the end of the recovery section 2c. Thedirection of coolant flow is from the entry point at 50 in the directionmarked by the arrow connecting the jacket elements to the dischargepoint at 51.

Equivalent provisions are made for small heat input to unscrapedsurfaces, and to compensate for conduction losses in the purifyingsection, as have been described for the first arrangement illustrated inFIGS. 1 and 2. A small evenly distributed heat input mayalso be appliedadvantageously to the unscraped surfaces of the passages between thesecylinders.

A third arrangement is illustrated by reference to FIG. 5. A refiningsection 61, a recovery section 62 and a purifying section 63 areassembled in line on a vertical axis. The feedstock entry point is at64, the liquid discharge point is at 66, and the melted solid productdischarge is at 67, below the heating element 68.

Since the crystals deposited are conveyed from the recovery sectionthrough the refining section to the purifying section by gravity, amodified scraper system 69, 70 is provided to maintain heat transferthrough the walls of the recovery and refining sections. To facilitateassembly a common shaft 71 driving the scrapers 69 and 70 and theagitator 73 is provided with articulating means at 72.

Coolant flow enters the recovery section jacket at 74, passes to therefining section jacket through the bridging connection 75, and isdischarged at 76. Here again, separate control of coolant flowtemperature to the jackets of. the refining and recovery sections may beof advantage.

This vertical form of the equipment of this invention operates with asubstantially continuous crystal bed extending from a level just abovethe heating element 68 in the purifying section upward through therefining section 61 and into the recovery section 62, the upper level ofthe crystal bed being a variable determined by variation of the heatinput and heat removals, the upper level of the crystal bed beingmaintained between the sight glasses or other level detecting devicesassociated with the inspection ports 77 and 78.

In the above described three arrangements of equipment, the crystal massreaching the base of the purifying section is melted by heating meansprovided within the base of the purifying section to allow withdrawal ofpart of the melt as product.

While FIGS. 1, 3 and 5 as presented show recovery sections and refiningsections of approximately equal length, in practice this relation willbe varied. A feedstock rich in the desired higher melting component willin general require a shortened refining section and lengthened recoverysection. A feedstock poor in that material will require a shortenedrecovery section and a lengthened refining section. A crystal feedstockmay require a crystal-forming recovery section of minimum cross section.As the liquid component of the feedstock becomes weaker in the desiredhigh melting component, the recovery section becomes shorter, until whensaid liquid component reaches eutectic composition the recovery sectiondisappears to be replaced by some auxiliary crystal forming means. Animportant feature of the invention is that it can accept a feed of anyconcentration within the limits set by the phase diagram, in liquid,crystal or slurry form, and can make a complete separation within thesame limits.

A variation applicable to any one of these three arrangements isillustrated by the block diagram of FIG. 6. From the base of thepurifying section 80 crystal is withdrawn through a rotary valve orequivalent 81 and passes to a separating device 82 serving to separatecrystal 83 from adhering liquid 84. The crystal mass is divided by asplitter 85 into crystal product 86 which is drawn off and a returnfraction 87. The return fraction 87 passes to a heat exchanger 88 withheat input means 89 and is thereby melted. The resultant liquid,together with the liquid stream 84, passes by a return line 90 intothebase of the purifying section where it is distributed by a ring jetor equivalent to become the reflux liquid stream passing up through thecrystal mass in the purifying section in countercurrent.

EXAMPLE 1 An arrangement of the equipment of this invention in the formshown and described in relation to FIGS. 3 and 4 was used to prepareparadichlorobenzene from a feedstock consisting of mixeddichlorobenzenes containing 75 percent of the paraisomer.

The feed was run in at a rate of 60 gallons per hour, and thereflux/product ratio was maintained at 0.5: l.

A tails product containing 75 percent orthodichlorobenzenewas withdrawnat a rate of 20 gallons per hour l5 gallons DCB and 5 gallons p DCB).

A very pure paradichlorobenzene assaying 99.99 percent was withdrawn ata rate of 40 gallons per hour.

By application of the design principles described above temperature andquality gradients were established so that at no section of the unit wasthe liquid subjected to a cooling rate exceeding 3 C. per hour.

By adjustment of the weirs the retention time of crystals conveyed inthe refining and recovery sections in countercurrent to the liquid wasadjusted so that they increase in temperature at approximately the samerate of 3 C. per hour.

By this means crystals were produced of a size and quality such that aproduct rate of 35 gallons per hour per sq. ft. of gross purifyingsection cross section was consistently maintained when producing productof the stated purity at the stated reflux ratio, the average temperatureand quality gradient in .the purifying section being 0.6 C. per foot ofheight approximately. On an exponential basis the impurity content ofthe crystal stream was reduced by 50 percent for each foot of purifyingsection height.

On a theoretical basis theutility heat required to effect the abovealmost complete separation within the limits set by the eutectic percentparadichlorobenzene and 85 percent orthodichlorobenzene) is thatrequired to provide latent heat to melt the product and the reflux,where a molten product is taken off, and that required to melt thereflux only where the product is taken off in crystal form and also inboth cases the sensible heat to raise the product temperature from feedtemperature to that of the melting point of product material. Apractical low-temperature limitation in the coolant refrigerationapparatus alone prevented the complete removal of paradichlorobenzenedown to the eutectic.

The heat removed by the coolant is the latent heat required tocrystallize the product and the reflux, and to cool the outgoing coldend impurity stream from feed temperature to that of the outgoingstream.

Additional heat is required in practice to maintain the temerature'ofall internal and external unscraped surfaces to be just above that ofthe crystallizing point of the adjacent liquor.

Extra cooling is required for the removal of this additional heat. Extraheating and cooling is also required to offset normal heat losses orgains through external insulation. in a unit of the capacity indicatedan efficiency exceeding 50 percent of the theoretical basis describedabove was obtained.

Because of the extremely low speed of the scraper-conveyors in therefining and the recovery sections the power consumption in theapparatus is extremely low, the connected power being 1.5 hp. permillion pound per annum of product including that required for the feedpump and the coolant circulating pump. This does not include powerassociated with the supply of refrigerant to cool the circulatingcoolant.

EXAMPLE 2 The plant employed in Example 1 was caused to operate underdifferent rates of feed and product withdrawal.

95 gaL/hr.

Feed rate 76 gaL/hr.

Product rate 54 gaL/hr. 60 gaL/hr. Reflux ratio 0.25 to l 0.25 to lProduct set point 53.l C. 510 C. Product quality 99.6% 99.5%

The results indicated that quality was beginning to be sacrificed togive the throughput obtained. At the higher throughput the thermalefficiency, expressed as the ratio of the sensible heat and latent heatremoved, including reflux heat, to the total heat removed exceeded 60percent.

EXAMPLE 3 A preliminary experimental run was undertaken with the solidsolution system paradibromobenzene paradichlorobenzene, using a form ofequipment with abbreviated recovery and refining sections. Under totalreflux steady state conditions were obtained in eleven hours.

Sample analyses showed the paradibromobenzene content to be as followsat the sites indicated:

Feed end, refining section 64.4% wt.

Discharge end, refining section 7l.47 wt.

Base. purifying section zone and refining zone each having decreasingcross-sec-.

tional areas in the direction of liquid flow; b. extracting heatcontinuously throughout the length of said recovery zone and refiningzone to generate a con-' tinuous downward temperature gradient from thejunc tion of the refining zone with the purifying zone to the end of therecovery zone remote from the purifying zone; c. controlling the axialliquid phase velocity at any point in a direction opposite to thedirection of crystal movement to generally exceed the liquid dispersionor back-mixing velocity in the opposite direction caused by agitatormovement and changes in liquid density by employing in conjunction boththe aforesaid variation of cross-sectional area available for crystaland liquid flow as the mass rate of flow of these streams vary along thetemperature gradient and also the relationship of feed, reflux, andproduct withdrawal rates; maintaining the bed of crystals insuspensionin the liquid phase in each of said recovery, refining and purifyingzones in a state intermediate between sedimentation and fluidization bycontrol of speed of stirrer and conveying means and liquid flowvelocity;

transporting the crystals formed in the recovery zone slowly through therefining zone to the purifying zone to maximize equilibrating contact ofcrystal with the countercurrent flow of liquid at all points;

f. maintaining a slow rate of temperature change of the crystal andliquid streams as they pass through the recovery and refining zones toproduce a continuing increase in the size of crystals as they move alongthe upward temperature gradient towards the purifying zone, thusminimizing the reflux required for purification in that zone and theassociated heating and cooling requirements;

g. withdrawing portion of the melted crystals produced by the heatingelement in the purifying zone as pure product, and passing the balanceas liquid reflux through the purifying zone and in diminishing quantitythrough the refining and recovery zones, and withdrawing a mother liquorat the end of the recovery zone remote from the purifying zone.

2. A process according to claim 1 wherein the change in themulticomponent mixture between its crystalline solid state and itsliquid state is a change from liquid state to crystalline solid state. i

3. A process according to claim 1 wherein the change in themulticomponent mixture between its crystalline solid state and itsliquid state is a change from crystalline solid state to liquid state. v

4. The process according to claim 1 in combination with the step ofadding selective heat inputs to the respective zones sufficient toprevent crystal agglomeration.

5. Apparatus for the separation, purification and recovery of a at leastone selected component of a multicomponent mixture of materials,comprising, in combination,

a. first enclosure means defining a recovery zone having increasingcross sectional areas from a first end to a second end,

b. second enclosure means defining a refining zone having increasingcross sectional areas from a first end which communicates with thesecond end of the enclosure means defining said recovery zone to asecond end,

c. third enclosure means defining a purifying zone having a first endcommunicating with the second end of the enclosure defining saidrefining zone and a second end,

d. stirrer means disposed substantially throughout the respectivelengths of each of said enclosure means,

e. heating element means in said third enclosure means disposed adjacentthe second end thereof,

inlet means for introducing a multicomponent mixture into the apparatusat a point adjacent the juncture of the second end of said firstenclosure means and the first end of said second enclosure means,

g. selected component outlet means at the second end of said thirdenclosure means,

h. fluid outlet means at the first end of said first enclosure means,

i. cooling means operably connected to said first and said secondenclosure means adapted to provide a downward temperature gradient alongthe respective lengths thereof from each said respective second endthereof towards each said respective first'end thereof, and

j. means for providing a heat input into said stirrer means sufficientto prevent crystal aecumulation thereon 6. Apparatus according to claimwherein the stirrer means in the first enclosure means and the stirrermeans in the second enclosure means are adapted to move crystals thereinaway from each of said respective first ends towards each of saidrespective second ends.

7. Apparatus according to claim 6 wherein the first enclosure means andthe second enclosure means are arranged horizontally and the thirdenclosure means is arranged vertically.

8. Apparatus according to claim 7 which includes acrystalmovement-restraining weir at the internal junction between thesecond end of the second enclosure means and the first end of the thirdenclosure means. V v

9. Apparatus according to claim 7 which includescrystalmovement-restra'ining weirs at the internal junctions betweeneach element of said first enclosure means and said second enclosuremeans.

10. A process in accordance with claim 1, wherein a small positive heatinput is provided to all unscraped surfaces in said recovery andrefining zones sufficient to prevent crystal buildup on said surfaces,and a small heat input to the stirrer and walls of the said purifyingzone throughout its length in order at least to keep the temperature. ofsaid stirrer and walls just above the crystallizing point of the liquidimmediately adjacent thereto. a

11. A process for the separation of at least onelco'mponent of amulticomponent mixture comprising:

a. feeding the mixture into. a solid-liquid continuouscountercurrentp'urifier cbmprising in combination a recovery zone, arefining zone and apurifying'zon c, said recovery.

zone and refining zone each being d isposed horizontally and havingdecreasing cross-sectional areas in the direction of liquid flow; g

b. extracting' h'eat continuously throughout the length of said recoveryzone and refiningfzone to generate a continuous downward temperature;gradient from the iunction of the refining zone with purifying zone tothe end of the'recovery zo'ne remote from the purifying zone;

c. controlling the axial liquid phase velocity at any point in adirection opposite to the direction of crystal movement to generallyexceed the liquid dispersion or back-mixing velocity in the. oppositedirection caused by agitator S movement and changes in liquid density byemploying in conjunction both the aforesaid variation of cross-sectionalarea available for crystal and liquid flow as the mass rate of flow ofthese streams vary along the temperature gradient and also therelationship of feed, reflux, and product withdrawal rates; I d.maintaining the bed of crystals in suspension in the liquid phase ineach of said recovery, refining and purifying zones in a stateintermediate between sedimentation and fluidization by control of speedof stirrer and conveying means and liquid flow velocity; I e. definingthe depth of the bed of crystals in suspension in the liquid phase ineach of said recovery and refining zones; f. transporting the crystalsformed in the recovery zone slowly through the'refining zone to thepurifying zone to maximize equilibrating contact of crystal with thecountercurrent flow of liquid at all points; g. maintaining a slow rateof temperature change of the crystal and liquid streams as they passthrough the recovery and refining zones to produce a continuing increasein the size of crystals as they move along the upward temperaturegradient towards the purifying zone,

thus minimizing the reflux required for purification in that zone and te associated heating and cooling requirements;

h. withdrawing portion of the melted crystals produced by the heatingelement in the purifying zone as pure product,

and passing the balance as liquid reflux through the purifying zone andin diminishing quantity through the refining and recovery zones, andwithdrawing a mother liquor at the end of the recovery zone remote fromthe purifying zone.

[2. A process in accordance with claim ll, wherein a small positive heatinput is provided to all unscraped surfaces in said recovery andrefining zones sufficient to prevent crystal buildup on said surfaces,and a small heat input to the stirrer and walls of the said purifyingzone throughout its length in order at least to keep the temperature ofsaid stirrer and walls just above the crystallizing point of the liquidimmediately ad- 5 .jacent thereto.

2. A process according to claim 1 wherein the change in themulticomponent mixture between its crystalline solid state and itsliquid state is a change from liquid state to crystalline solid state.3. A process according to claim 1 wherein the change in themulticomponent mixture between its crystalline solid state and itsliquid state is a change from crystalline solid state to liquid state.4. The process according to claim 1 in combination with the step ofadding selective heat inputs to the respective zones sufficient toprevent crystal agglomeration.
 5. Apparatus for the separation,purification and recovery of a at least one selected component of amulticomponent mixture of materials, comprising, in combination, a.first enclosure means defining a recovery zone having increasing crosssectional areas from a first end to a second end, b. second enclosuremeans defining a refining zone having increasing cross sectional areasfrom a first end which communicates with the second end of the enclosuremeans defining said recovery zone to a second end, c. third enclosuremeans defining a purifying zone having a first end communicating withthe second end of the enclosure defining said refining zone and a secondend, d. stirrer means disposed substantially throughout the respectivelengths of each of said enclosure means, e. heating element means insaid third enclosure means disposed adjacent the second end thereof, f.inlet means for introducing a multicomponent mixture into the apparatusat a point adjacent the juncture of the second end of said firstenclosure means and the first end of said second enclosure means, g.selected component outlet means at the second end of said thirdenclosure means, h. fluid outlet means at the first end of said firstenclosure means, i. cooling means operably connected to said first andsaid second enclosure means adapted to provide a downward temperaturegradient along the respective lengths thereof from each said respectivesecond end thereof towards each said respective first end thereof, andj. means for providing a heat input into said stirrer means sufficientto prevent crystal accumulation thereon.
 6. Apparatus according to claim5 wherein the stirrer means in the first enclosure means and the stirrermeans in the second enclosure means are adapted to move crystals thereinaway from each of said respective first ends towards each of saidrespective second ends.
 7. Apparatus according to claim 6 wherein thefirst enclosure means aNd the second enclosure means are arrangedhorizontally and the third enclosure means is arranged vertically. 8.Apparatus according to claim 7 which includes acrystal-movement-restraining weir at the internal junction between thesecond end of the second enclosure means and the first end of the thirdenclosure means.
 9. Apparatus according to claim 7 which includescrystal-movement-restraining weirs at the internal junctions betweeneach element of said first enclosure means and said second enclosuremeans.
 10. A process in accordance with claim 1, wherein a smallpositive heat input is provided to all unscraped surfaces in saidrecovery and refining zones sufficient to prevent crystal buildup onsaid surfaces, and a small heat input to the stirrer and walls of thesaid purifying zone throughout its length in order at least to keep thetemperature of said stirrer and walls just above the crystallizing pointof the liquid immediately adjacent thereto.
 11. A process for theseparation of at least one component of a multicomponent mixturecomprising: a. feeding the mixture into a solid-liquid continuouscountercurrent purifier comprising in combination a recovery zone, arefining zone and a purifying zone, said recovery zone and refining zoneeach being disposed horizontally and having decreasing cross-sectionalareas in the direction of liquid flow; b. extracting heat continuouslythroughout the length of said recovery zone and refining zone togenerate a continuous downward temperature gradient from the junction ofthe refining zone with the purifying zone to the end of the recoveryzone remote from the purifying zone; c. controlling the axial liquidphase velocity at any point in a direction opposite to the direction ofcrystal movement to generally exceed the liquid dispersion orback-mixing velocity in the opposite direction caused by agitatormovement and changes in liquid density by employing in conjunction boththe aforesaid variation of cross-sectional area available for crystaland liquid flow as the mass rate of flow of these streams vary along thetemperature gradient and also the relationship of feed, reflux, andproduct withdrawal rates; d. maintaining the bed of crystals insuspension in the liquid phase in each of said recovery, refining andpurifying zones in a state intermediate between sedimentation andfluidization by control of speed of stirrer and conveying means andliquid flow velocity; e. defining the depth of the bed of crystals insuspension in the liquid phase in each of said recovery and refiningzones; f. transporting the crystals formed in the recovery zone slowlythrough the refining zone to the purifying zone to maximizeequilibrating contact of crystal with the countercurrent flow of liquidat all points; g. maintaining a slow rate of temperature change of thecrystal and liquid streams as they pass through the recovery andrefining zones to produce a continuing increase in the size of crystalsas they move along the upward temperature gradient towards the purifyingzone, thus minimizing the reflux required for purification in that zoneand the associated heating and cooling requirements; h. withdrawingportion of the melted crystals produced by the heating element in thepurifying zone as pure product, and passing the balance as liquid refluxthrough the purifying zone and in diminishing quantity through therefining and recovery zones, and withdrawing a mother liquor at the endof the recovery zone remote from the purifying zone.
 12. A process inaccordance with claim 11, wherein a small positive heat input isprovided to all unscraped surfaces in said recovery and refining zonessufficient to prevent crystal buildup on said surfaces, and a small heatinput to the stirrer and walls of the said purifying zone throughout itslength in order at least to keep the temperature of said stirrer andwalls just above the crystallizing point of the liquid immediatelyadjacent thereto.